He will be 18 in December,he's in the 10th grade,going to 11th maybe,he was born with alcohol fetal syndrome and is mentally incompetent.He lived with me from the day he was born until 3 or 4 yrs old.He was born in the state I live in.
The young person seems as if he would benefit from counseling, therapy, a psychologist, a psychiatrist, or all of the aforementioned.
You might also seek a support group, counseling, and therapy before you embark upon this adventure.
Some people can't ever function in society.
Some people need to be in hospital or supervised care facilities.
I don't know the young man, and he'll be an adult around Christmas time anyway.
Even if you sought custody, it would take six to seven months to achieve and would be expensive.
Perhaps your money and effort would be better spent by helping the young man and his guardian with counseling, therapy, etc...
Once he becomes an adult, you'll need to become a guardian.
I suggest you discuss this matter with a support group for mentally challenged or emotionally maladjusted teenagers.
I wish you all your heart desires, but be smart.